This invention relates to a low wattage, tungsten-halogen lamp and, in particular, to a low wattage lamp having improved performance, high efficacy and long life.
As explained in U.S. Pat. Nos. 4,163,171 and 4,451,760, many incandescent lamps employ the well-known tungsten-halogen regenerative cycle. At relatively high operating temperatures, tungsten atoms are evaporated from the filament. These atoms are captured by the halogen gas and return to the filament to replace lost material. A reduction in size of these lamps produces a dramatic increase in thermal loading and a reduction in lamp life and maintenance. Maintenance, as herein used, is defined as the percent of initial lamp light output that is maintained over the life of the lamp. Output, as herein used, is defined as the visible light, 400 to 700 nanometer wavelength, emanating from the lamp. Typically, lamps in the T-1 to T-11/4 range exhibit a marked drop off in maintenance after 15 to 20 hours of operation and generally fail rapidly thereafter.
U.S. Pat. No. 4,598,342 discloses a low wattage, tungsten-halogen lamp that employs a double filament arrangement. The filaments are aligned end to end along the axis of the lamp envelope and each is arranged to operate independently of the other. The coils in each filament are spaced further apart at the center of the filament than at each end to provide for a more uniform temperature distribution along the length of each filament. The double filament arrangement increases the thermal loading on the lamp when both filaments are operating. To compensate for the higher than normal loading, the fill gas pressure in the lamp is increased anywhere from 2 to 20 atmospheres. The fill gas contains Xenon which is said to maintain the inactive filament at a cooler temperature (i.e. the one filament is turned off). This prevents halogen in the fill gas from attacking the filament components. The patent does not disclose the fractional percentage of halogen gas contained in the Xenon gas.